Three-phase slurry reactors, comprising a mixture of solid particles and at least one liquid phase forming together at least one suspension, and a gas phase flowing upwards through the suspension, are well known to those skilled in the art. In these reactors, solid catalyst particles are dispersed or held in suspension in the liquid phase by a gas phase bubbling through the liquid phase. In operation, said reactors typically comprise a slurry zone and a freeboard zone, the slurry zone comprising the solid particles in suspension in the liquid, gaseous reactants bubbling through the slurry, and the freeboard zone, located above the slurry, comprising primarily the gaseous products and/or reactants.
Examples of chemical processes which are carried out in a three phase slurry reactor are those which make use of solid catalyst particles and of at least one gaseous reactant and produce a product which is liquid under reaction conditions. Examples of such processes include hydrogenation processes, hydroformylation, alkanol synthesis, the preparation of aromatic urethanes using carbon monoxide, Kölbel-Engelhardt synthesis, polyolefin synthesis, and Fischer-Tropsch synthesis.
Many ways have been proposed to separate at least part of the liquid from the multiphase mixture.
For example, in EP 0609079, a filtration zone is located in the slurry bed, close to its upper surface. The liquid product is separated from the solid particles by passing through a filtration medium in a first direction, so that a cake of the solid particles forms on the filtration medium. However, such a dead-end filtration implies regular backflushing of the filtering medium in a second direction, opposite to the first one, to dislodge the cake from the filtering medium. The liquid used for backflushing constitutes an additional load which will also have to be filtered. The productivity of the global process will thus be lowered due to backflushing.
In WO 94/16807, the filtration zone surrounds the reactor vessel and the filter element may be provided by a portion of the wall of the reactor vessel itself, which is composed of a filter material. With such a design, there is no build-up of solid material on the filter element, due to the turbulent motion of the slurry. Yet, such a filtering system is elaborate and is thus expensive and difficult to implement. Furthermore, such an internal filtering system implies the shut down of the vessel for maintenance, for example if the filter medium needs chemical cleaning.
U.S. Pat. No. 5,900,159 discloses the degasification of the multiphase mixture using a hydrocyclone or a specific continuous disengagement method separation and the subsequent separation of the resulting slurry into the liquid and a concentrated slurry via a cross-flow filter located outside the vessel, said slurry being brought to the cross-flow filter through the medium of a pump. The main advantage of the cross-flow filtration system is to avoid the build-up of solid material on the filter medium.
Cross-flow filtration is a well known filtration method, wherein the residue (retentate) is continuously removed from the filter medium by shear of the slurry which flows along the filter, in tangential flow to the filter medium. The shear can be produced by rotating elements such as rotating filters or rotors, but the shear is usually produced by the relative velocity of the slurry to the filtration medium. A general overview of cross-filtration can be found in Kirk-Othmer Encyclopedia of Chemical Technology (2003), Chapter “Filtration”, pages 383-388 (DOI 10.1002/0471238961.0609122019220118.a01.pub2), which is incorporated herein by reference.
The method of U.S. Pat. No. 5,900,159 requires the use of a pump between the degasification mean and the cross-flow filter. A first drawback associated to the use of said pump is to lead to at least some attrition of the solid particles or, if the particles are not sensitive at all to attrition, to erosion of the pump. A second drawback is that the pump is sensitive to gas and tends to function less properly in the presence of gas, which implies a perfect degasification of the slurry upstream of the pump. A third drawback is that pumps consume energy.